Pettitte, who will start against the Braves in Game 1 of the National League Division Series on Wednesday in Atlanta, went undefeated in September and helped lead the Astros to their second straight Wild Card.
The left-hander's 4-0 record and 1.86 ERA in six starts last month helped him to pick up his second National League Pitcher of the Month Award of the season.
"A lot of people had said, 'If you had a Game 7 of a World Series to pitch, he'd be the guy,'" Houston second baseman Craig Biggio said last month. "Obviously now, after playing with the guy for two years, I believe it.
"The man can pitch."
Pettitte struck out 29, walked four and allowed just 27 hits in 38 2/3 innings last month. He picked up no-decisions in his final two starts, but gave up only one earned run in each of those outings.
His win against Philadelphia on Sept. 5 came in the first contest of a three-game sweep over Philadelphia, who finished one game behind Houston in the Wild Card standings. The Astros went 17-11 in September to clinch the final postseason spot.
Pettitte's final month capped off an impressive second half for the southpaw. He went 13-2 since the start of July and finished with a 17-9 record and a 2.39 ERA in 33 starts for the season. It marked the most wins for Pettitte since collecting 21 victories in 2003. It was the fifth time in his career that Pettitte won 17 or more games.
Last year, in his first season with Houston, Pettitte was limited to just 15 starts. He finished the year 6-4 and had to undergo season-ending surgery on his left elbow.
This season, though, Pettitte pitched 222 1/3 innings, his highest total since appearing in 240 1/3 in 1997 with New York.
"I played with the Yankees for nine years and I made my starts," Pettitte said last month. "If you make 34 starts with that team, you're going to win a lot of ballgames. I feel like if I stay healthy and make my starts for any team, I should be able to win. It's good to continue to win."
The award was the fourth that Pettitte has won in his career. He was selected over Ryan Dempster (0-0, 0.00 ERA, 13 saves) of the Cubs; Philadelphia's Jon Lieber (4-1, 1.76 ERA, 29 strikeouts); Derrick Turnbow (2-0, 1.64 ERA, 10 saves) and Rick Helling (2-1, 1.69 ERA, 32 strikeouts) of Milwaukee; Tom Glavine (3-2, 1.76 ERA, 34 strikeouts) of the Mets; and Washington's Hector Carrasco (2-0, 0.95 ERA, 31 strikeouts).
Jordan Bastian is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.